1. Technical Field of the Invention
The invention relates to production of irons and steels, and additives used in such production.
2. Description of the Related Art
The usual microstructure of gray iron is a matrix of ferrite and pearlite with graphite flakes dispersed throughout. Foundry metallurgical practices include inoculating the metal so that nucleation and growth of graphite flakes occurs in a pattern that enhances the desired mechanical properties, by addition of an inoculating. An inoculating agent can be added to either 1) the pouring ladle, 2) injecting or spraying the inoculant (in a finely divided granular or powdered form) into the metal pouring stream as the molten metals enters the mold, or as a insert placed in the mold. The amount, size and distribution of graphite are important to the physical properties of the gray iron. The use of inoculants to control microstructure as well as reduce the chilling tendency or the formation of iron carbides (or cementite) is common practice in the ferrous foundry industry. The presence of iron carbides in the iron matrix is undesirable because this constituent is hard and brittle and can result in poor mechanical properties and machinability.
In ductile irons, the usual microstructure is a matrix of ferrite and pearlite with graphite nodules dispersed throughout the structure. The size, shape and distribution of the graphite nodules is important to the physical properties of the ductile iron. Similar to gray cast iron, the nucleation and growth of the graphite nodules can be controlled by adding materials referred to as “post inoculants,” to either the ladle, as an instream inoculant or as an insert placed at a strategic location in the mold. In some cases, alloy additions of silicon carbide, pure graphite and other proprietary alloy mixtures are added to molten ductile irons prior to magnesium treatment. This practice is referred to as preconditioning the molten iron.
Inoculants can best be described as elements that offer the possibility to form stable compounds with either, sulfur or oxygen, or with both. These oxy-sulfide atomic clusters provide a substrate surface with nucleation sites upon which dissolved graphite in the molten iron can start to grow as graphite flakes or nodules, before sufficient undercooling occurs that favors the formation of carbides.
Numerous metals and alloys have been proposed for use as inoculating agents in the production of both gray and ductile iron castings. Standard inoculating agents are 1) calcium silicon, 2) calcium bearing ferrosilicon alloys or other ferrosilicon based alloys that contain small percentages of oxy-sulfide forming elements and 3) finely divided and powdered synthetic graphite. The oxy-sulfide forming elements contained in inoculants are cerium and other rare earths, zirconium, calcium, aluminum, barium, strontium, magnesium and titanium. These may be referred to as “oxy-sulfide formers,” individually or in selected amounts.
The effectiveness of all inoculating agents is a direct function of the amount of sulfur dissolved in the molten irons and to a lesser extent, the amount of dissolved oxygen. The ability of oxy-sulfide forming elements to form nuclei assisting substrates, i.e., oxy-sulfide atomic clusters, which in turn provides a similar crystalline surface onto which dissolved graphite atoms can precipitate from the liquid iron and grow is a necessary prerequisite for inoculation. Addition of these sulfide and oxygen compounds rejuvenates and beneficiates the molten iron and improves its responsiveness to inoculation.
It has been observed with cast irons held in a holding furnace overnight or over a weekend, insufficient oxygen and “stale” or “unreactive sulfur” levels result and as a result, these irons do not respond well to traditional inoculation methods. Sulfur and oxygen are not readily available to react with the oxy-sulfide elements to form suitable substrates for inoculation. More recently, it has been found that a joint addition of both oxygen and sulfur compounds to molten cast irons will improve the performance of cast iron inoculants. Resulfurizing cast irons with “fresh” sulfur in, conjunction with the simultaneous addition of oxygen compounds, enables improved inoculation. Fresh additions of new sulfur and oxygen allow the oxy-sulfide containing elements in inoculants to more easily form substrates for graphite precipitation.
In a few instances, intentional incorporation of sulfur and oxygen containing elements are made to the surface of ferrosilicon based inoculants. It has been claimed that the controlled additions of the elements sulfur and oxygen to commercially available ferrosilicon based inoculants can provide enhanced performance. This is an attempt to ensure that sufficient sulfur and oxygen will be available for subsequent reactions with the oxy-sulfide elements added as inoculants. However, in practice, the coating of oxygen and sulfur containing particles on the surfaces of ferrosilicon particles is not sufficiently concentrated to allow improved performance. Another drawback of incorporating inoculants which contain fine oxygen and sulfur particles coated onto larger grains of ferrosilicon inoculants is a rather unsightly product that is subject to particle segregation and coating failure and coating de-lamination. During shipping of such ferrosilicon inoculants, segregation and separation of sulfur and oxygen particles occurs, negating their final effect.
Therefore it will be appreciated that improvements in such additives would be desirable.